This invention relates to a braking force control apparatus for optimizing wheel braking of respective road wheels, separately.
For example, Japanese Patent Kokai Nos. 53-131382 and 4-151354 disclose braking force control apparatus of the type where the braking forces applied to the traction or drive road wheels that are driven by the vehicle engine are controlled independently of the follower or driven wheels that are not driven by the vehicle engine. These conventional apparatuses intend to ensure that the traction road wheels have the same braking efficiency as the follower road wheels by preventing the engine brake from being applied to the traction road wheels thereby keeping the skidding control performance of the traction road wheels free from the influence of the engine brake while the skidding control is performed to prevent the road wheels from being locked. With such conventional apparatus, however, a creep torque will act on the traction road wheels in the direction opposite to the direction of the engine brake torque thereby reducing the braking efficiency for the traction road wheels when the vehicle is being braked at low speeds with the accelerator pedal released. This tendency is increased particularly for vehicles of the type including an automatic transmission through which a drive torque is transmitted from an engine to the traction road wheels since a shift down occurs in the automatic transmission, causing a great change of traction wheel drive torque during travel of the vehicle at low speeds. Furthermore, the conventional apparatuses exert greater load on the associated road wheel braking system since the engine brake is not utilized.
It is proposed, in Japanese Patent Kokai No. 6-32217, to increase the road wheel braking force to compensate for the road wheel braking force reductions when a creep torque is produced and, in Japanese Patent Kokai No. 7-81463, to decrease the road wheel braking force to compensate for the road wheel braking force increases when an engine brake torque is produced. With these proposals, however, the braking force is decreased or increased uniformly for all of the road wheels. For example, if the braking force is increased uniformly for all of the road wheels when a creep torque is produced, excessive braking force will be applied to increase the tendency of the follower road wheels, which are free from the influence of the creep torque, to be locked, whereas braking fore insufficient for the creep torque will be applied to the traction road wheels. Such inbalanced application of braking force occurs not only between the traction and follower road wheels, but also between the left and right road wheels. This is true particularly when the vehicle is travelling on a road surface having a low friction factor or when the vehicle is turning.